The overall objective is to formulate methods for determining the best age for vaccination against measles in diverse geographic areas, in the presence of reduced virus circulation due to partially successful programs, and in infants whose mother were vaccinated. Specifically, we will: 1) Develop a more reproducible assay for measles immunity. 2) Determine effects of low levels of maternal antibody on titer of persistent antibody in vaccinated children. 3) Define the correlation between maternal antibody titer and infant response to vaccin in diverse populations. 4) Characterize the effects of vaccine immunized mothers on infant susceptibility. 5) Test the effects of revaccination in children whose response to a first dose was blocked by maternal antibody. These studies will be carried out in the U.S.A. and in developing countries of South America. A long term goal is to improve vaccination programs in developing countries for their own protection and to reduce the frequency of virus importation to the U.S.A.